03. People use many sources of energy. Ancient people used only their arms, hands, and simple
tools until they discovered fire. Later, people learned to use the energy of the wind to sail ships.
Then they learned to use animals to do work. In the eighteenth and nineteenth centuries, people
began to use machines. The first machines used steam. They burned coal to heat water. The hot
water turned into steam, and the steam engine did the work. Later, people began to use the
gasoline engine, and today people are using nuclear energy.
Energy is one of the basic concepts, or ideas, of physics. It is the ability to do work. Without
energy we could live. We need energy to work, and scientists are always looking for new sources of
energy.
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